After the completion of Hoover Dam in 1935, sediment began accumulating in the new reservoir (Lake Mead) as the flow of the Colorado River was captured. Three studies have examined the amount of sediment accumulation and the rate of sedimentation in Lake Mead: a 1948-1949 USGS and Reclamation study (Smith et al. 1960), a 1963-1964 Reclamation study after the closure of Glen Canyon Dam (Lara and Sanders 1970), and a 2001 USGS and Reclamation study. Results of the 2001 study show that post-impoundment sediment is generally distributed along the floors of the deepest parts of the lake, mainly following the paths of the former Colorado River and the tributary valleys that fed into it, rather than being deposited as a drape across the entire lake floor (Twichell et al. 2005). These sediments are thickest in the deltas that formed at the mouths of the Colorado River and its tributaries, including the Virgin and Muddy Rivers. Maximum sediment thickness exceeds 262 ft where the Colorado River enters Lake Mead, thinning to 50-115 ft in thickness along the remainder of the drowned Colorado River channel to Hoover Dam. Tributary valleys have a thinner sediment cover indicating the Colorado River has been the primary sediment source. Sediment cores indicated stratification of fine silt interrupted by graded beds containing as much as 30 percent sand deposited from turbidity currents, which flowed the full length of the lake. With the completion of Glen Canyon Dam in 1964, sediment volume entering Lake Mead from the Colorado River decreased to approximately one tenth of the pre-dam volume (Lara and Sanders 1970). With the lake levels dropping since 2000, delta deposits at the mouth of the Colorado River and tributaries, including Las Vegas Wash, have been eroded by the river flow and redistributed to the deeper parts of the lake.

Sediment cores taken in 1998 have been examined for anthropogenic and natural organic and inorganic contaminants (Covay and Beck 2001; Rosen and Van Metre 2009). In addition, sediment from Las Vegas Wash (the main tributary from Las Vegas) has also been examined for contaminants (Covay and Leiker 1998). These studies found numerous organic compounds associated with urban runoff, industrial contaminants from erosion of the Basic Management Incorporated (BMI) site onLas Vegas Wash, and compounds associated with tertiary treated wastewater effluent, although few compounds were greater than Canadian sediment quality guidelines (Rosen and Van Metre 2009).

In contrast to Lake Mead, remarkably little sediment has accumulated in Lake Mohave since its impoundment in 1953 (Foster 2004). Lake Powell (within Glen Canyon National Recreation Area) and other upstream reservoirs trap virtually all of the sediment transported by the Colorado River. The small amount of fine-grained sediment, which has accumulated, tends to occur in the deepest parts of the lake within sheltered areas along the edges of the drowned Colorado River channel. Other post-impoundment deposits include debris flows found at the mouths of washes probably associated with flash floods and landslides along the base of steep cliffs in the northern section of the lake, which appear to be the result of cliff collapse (Foster 2004). Knowledge of any contaminants present LakeMohave sediments is currently lacking.

Strategic Fundamental Objectives

A healthy sports fishery

Healthy populations of native fish

Healthy populations of aquatic dependent wildlife

Healthy shoreline dependent native vegetation

Existing high quality setting for water-based recreation

Regional and community needs for municipal and industrial uses, including domestic water supply and Colorado River System return flow credits

Management questions best answered by monitoring:

What is the status and trend of re-suspension and transport of contaminants and nutrients from sediments?

What is the status and trend of sediment delivery at tributaries?

What is the status and trend of contaminants in sediments? (See also Category 3).more